For A B and C, you just plug in the given number
A
[tex]2(15) + 5 = 35[/tex]
B
[tex]4(20) + 2 = 82[/tex]
C
[tex]6(10) + 5 = 65[/tex]
And for D, you set the equation to 26 and solve for n
[tex]3x + 5 = 26 \\ 3x = 21 \\ x = 7[/tex]
I had to use x instead of n, but for D n=7. :)